Many conventional ultrasound scanners create two-dimensional images of tissue located in a region of interest in which brightness of a pixel in the image is based on intensity of echo return following the provision of wave energy directed towards the region of interest. Such images may be formed from a combination of fundamental and harmonic signal components, the former being direct echoes of the transmitted pulse, and the latter being generated in a non-linear medium, such as tissue, from finite amplitude ultrasound propagation. As is known, many times, ultrasound images can be improved by suppressing the fundamental and emphasizing the harmonic signal components.
Propagation of ultrasound wave energy in biological tissues is known to be non-linear, giving rise to generation of harmonics. In harmonic imaging, energy is transmitted at a fundamental frequency, f0, and, for example, an image may be formed using energy at the second harmonic, 2f0.
Further, generally, in many instances, ultrasound contrast agents have been used for ultrasound imaging, e.g., imaged by using standard echo imaging or second harmonic imaging. Harmonic imaging is usually preferred over standard echo imaging when contrast agents are present because, for example, the harmonic signal components returned from contrast agents is generally much larger than that from surrounding tissue. Furthermore, for example, harmonic imaging provides a more desirable contrast between blood and tissue, and is able to reduce artifacts due to phase aberrations in the body. However, since harmonic imaging still receives signal from tissue, the specificity between contrast agent and tissue is still limited.
The diagnostic applications of ultrasound imaging have expanded enormously in recent years. Various improvements of ultrasound imaging as a diagnostic technique for medical decision-making have been established. Some of these improvements were with regard to ultrasound hardware/equipment, such as phased array transducers. Other improvements have included the introduction of signal processing algorithms that produced image enhancements, or more even, new forms of imaging such as color flow Doppler imaging.
Various methods to exploit the non-linear nature of ultrasonic propagation in tissue media are being used in an attempt to provide improved imaging techniques for enhancing ultrasonic imaging, with or without the use of contrast agents. For example, second harmonic imaging improves the image contrast by significantly reducing the acoustic clutter from intervening tissue layers. This is particular advantageous for difficult to image patients.
The simplest implementation of second harmonic imaging is the use of a post-beamforming bandpass filter to separate the second harmonic from the fundamental. The assumption is that if the transmitted imaging pulse is carefully designed to have frequency components in the band f0−B/2 to f0+B/2, then second-order linear effects produce new frequency components in the band 2f0−B to 2f0+B. However, this technique puts significant constraints on the transducer bandwidth, f0−B/2 to 2f0+B. Significant signal loss occurs since most transducers are not capable of supporting such a bandwidth.
Various enhancements have been made to this simple implementation of second harmonic imaging. For example, a pulse inversion technique has been proposed and described in Simpson et al., “Pulse Inversion Doppler: A New Method for Detecting Non-linear Echoes from Microbubble Contrast Agents,” IEEE Trans. On Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 46, No. 2, (1999), and also M. A. Averkiou, “Tissue Harmonic Imaging,” 2000 IEEE Ultrasonics Symposium, Vol. 2, pp.1563-1572 (2000).
Further, other ultrasonic imaging techniques are moving rapidly towards employing post-beamforming filters combined with non-linear imaging modes. For example, in U.S. Pat. No. 6,290,647 B1 to Krishnan, entitled, “Contrast Agent Imaging with Subharmonic and Harmonic Signals in Diagnostic Medical Ultrasound,” issued Sep. 18, 2001, a combination of the results of linear filtering of harmonic and subharmonic components are used to produce improved contrast imaging. Further, another ultrasound imaging approach is described in an article by Haider, B. and Chiao, R. Y., entitled “Higher Order Non-linear Ultrasonic Imaging,” 1999 IEEE Ultrasonics Symposium, Vol. 2, pp 1527-1531 (1999).
However, such techniques and enhancements are not without their own limitations. For example, the approach of Haider and Chiao performs non-linear imaging by recognizing the non-linear behavior of the system as a static polynomial-type non-linearity. It does not recognize or take into consideration the dynamic behavior of the system.
Further, the approach by, for example, Haider and Chiao, and also the pulse inversion techniques, require the use of multiple transmits in the same direction for estimating coefficients of harmonic bases functions of the models utilized. When relying on the use of multiple transmissions, movement of the imaged region results in undesirable degradation of the image produced by the ultrasound system.